This proposal examines the hypothesis that: (1) catecholamines (dopamine and norepinephrine) have role in development that precedes neurotransmitter function and that (2) catecholamines are made in the absence of tyrosine hydroxylase (TH) through the activity of tyrosinase, the enzyme that catalyzes the synthesis of melanin pigments. The studies will focus on two newly created mutations: a tyrosine hydroxylase (TH)-depleted mouse on a pigmented background that the investigator made through homologous recombination in embryonic stem cells and TH depleted mouse on a tyrosinase -deficient background. Both mutations are embryonic lethal conditions. Neither type of TH-null embryo displays glyoxylic acid induced fluorescence indicating that catecholamines are absent. Both types of TH-null mice can be rescued in utero with a variety of agents and they live for several weeks after birth without further treatment. It was expected that the original TH-null pups would contain no catecholamines because TH is essential for synthesis. However, the original TH-null, which is on a pigmented background, does display glyoxylic acid induced fluorescence indicating the presence of catecholamines postnatally. In contrast, the tyrosinase-deficient TH-null pups do not have histofluorescence. This indicates that tyrosinase, which like TH uses tyrosine as a substrate may supply catecholamines in these extraordinary circumstances. Using histological and biochemical techniques, it is planned to learn whether catechol or other amines are formed in the TH-null untreated embryos and rescued pups and whether they are a consequence of tyrosinase activity. It is intended to uncover the fate of neurons that in a normal animal would contain catecholamines. Finally, the rescued TH-null tyrosinase-deficient mice will be used to study two developmental events in which catecholamines may participate as regulators of development. The investigator will examine pattern formation in the striatum to test the hypothesis that reciprocal interactions of dopamine and its targets regulate the development of the striatal mosaic. They will test the hypothesis that norepinephrine induces the differentiation factor that directs the switch from an adrenergic to a cholinergic phenotype in sympathetically-innervated sweat glands.